The present invention relates to antennas, and more particularly to antennas for radio-signal navigation systems, such as global positioning systems, where it is desirable to reduce the effects of multipath signals.
Satellite navigation systems include the global positioning system (GPS) and the global orbiting navigation system (GLONASS). The systems are used to solve a wide variety of tasks that relate to determining object position, object velocity, and precise time. Land surveying is an important application of receivers based on satellite navigation systems. Such receivers have many advantages compared to conventional devices for land surveying. For example, satellite-based surveying systems are more responsive, can operate in nearly all types of weather and at all times of the day, and can be used in areas which do not have line-of-sight conditions.
However, there are some drawbacks to satellite navigation systems. These systems typically receive signals from four or more satellites and extract timing information from the satellite signals. Using three-dimensional triangulation, the position coordinates of the antenna receiver element can be determined from the extracted timing information. There are many sources of error that enter into the extraction and triangulation process, which in turn cause errors in the computed coordinates. One large error source arises from the reception of reflected versions of the satellite signals. These versions are reflected from the ground and neighboring objects and have timing information which is different from that contained in the true satellite signals. The total signal received by the antenna and measured by the receiver will be a combination of the true satellite signal and the reflected versions, and the final timing information extracted by the receiver will be a combination of the timing information of the true signal with that of the reflected versions. The resulting error in the computed coordinates can be several meters for stand-alone processing, and several centimeters for differential GPS processing (DGPS).
Multipath errors can be addressed at the receiver level by including circuits which detect and reject or mitigate multipath signals. Multipath errors can also be addressed at the antenna level, where the reception of mulitpath signals by the antenna element is reduced. This is the area that the present invention is directed to.
Reducing the reception of multipath signals can be accomplished by constructing an antenna system that provides a good xe2x80x9cdown/up ratioxe2x80x9d (also known as the xe2x80x9cfront to back ratioxe2x80x9d). Such antenna systems typically use a large ground plane underneath the antenna element to define a horizontal antenna plane, and are constructed to strongly decrease signals received from below the horizontal antenna plane, and hence decrease the effect of multipath signals caused by the Earth""s surface and other objects underneath the antenna.
The xe2x80x9cdown/up ratioxe2x80x9d is one of the most important parameters of a radio-navigation antenna, and is very useful in describing the ability of the antenna system to suppress reflections from the ground. We give a brief description of the ratio here, and a more detailed explanation in Appendix B. Normally, an antenna system is mounted on a pole which is positioned over a target point, with the axis of the pole being substantially collinear with the direction of gravitational pull at the target point. We will refer to this direction of gravitational pull as the plumb-position axis. In this configuration, the ground plane of the antenna is perpendicular to the plumb-position axis, and parallel to the horizontal plane that extends from the target point to the horizon in all directions. Suppose that we have a true satellite signal incoming to the antenna element at an elevation angle xcex8 with respect to the horizontal plane. Since the true satellite signal is in the form of plane waves, it strikes the antenna ground plane at an angle xcex8 with respect to the plane of the ground plane, and it strikes the Earth""s ground at an angle xcex8 with respect to the horizontal plane. Some of the signal striking the Earth reflects off the Earth""s ground at an angle xcex8 with respect to the horizontal plane, and propagates toward the underside of the antenna system. The reflected signal also strikes the underside of the antenna system (usually the ground plane) at an angle of xe2x88x92xcex8 with respect to the plane of the antenna ground plane. This reflected signal propagates around the surface of the antenna system toward the antenna element at the top surface, and a portion thereof is received by the antenna element, along with the true satellite signal. The amount of the reflected signal that is received by the antenna element generally depends upon the angle xe2x88x92xcex8 (as measured with respect to the plane of the antenna ground plane). As can be seen from the above, the level of reflected signal received by the antenna depends upon two factors: one is the reflection coefficient from the Earth and the other is the antenna""s directivity. While the first factor depends on the Earth""s properties and the antenna""s location, the second factor is determined only by the properties of the antenna system. The second factor can be characterized in terms of the down/up ratio. The down/up ratio is the ratio of the signal reception of a signal directed toward the underside of the antenna system with angle xe2x88x92xcex8 and power level Po to the signal reception of a signal directed toward the topside of the antenna system with angle xcex8 and power level Po. Angle xcex8 is generally called the elevation angle.
In general, the down/up ratio of an antenna system is principally determined by size and shape of the ground plane. Ideally, a flat metal ground plane of infinite extent would provide perfect suppression of signals received from below the horizontal antenna plane. In practice, many antenna systems employ large ground planes to provide good down/up performance. Among them is the well known GPS xe2x80x9cChoke Rings,xe2x80x9d which are ground planes which comprise several concentric grooves formed on the top surface of the ground plane. They are widely used in high precision GPS/GLONASS applications and provide good multipath rejection performance. The typical diameter of the ground planes in these systems is on the order of 30 cm to 50 cm, and so their use in portable radio-navigation equipment is rather limited because of their bulky nature. They are most often used as part of the antennas for base stations.
For the rover stations, one would like to use microstrip antennas because of their small size and manufacturability. However, these antennas have poor down/up ratios, and have very little multipath suppression capability.
The present invention is directed to providing an antenna system which is compact, and yet has good down/up ratios and good multipath suppression.
Broadly stated, the present invention comprises a receiving antenna and a passive antenna disposed in close proximity to one another, with the signal received by the receiving antenna being provided for processing or transmission, without any significant direct coupling of the signal received by the passive antenna.
In preferred configurations, the two antennas are mounted back to back, with their ground planes facing one another, or with their antenna elements disposed on opposite sides of a common ground plane or common grounded enclosure.
The inventors have found that this structure greatly improves the down/up performance of microstrip antennas having small ground planes.
As an unexpected benefit, the inventors found that the bandwidth of the antenna system is significantly increased, thereby enabling the antenna system to receive both differential correction signals transmitted on the INMARSET frequencies (1530 MHz) and the global positioning satellite signals (1560 MHz to 1610 MHz).
In another aspect of the present invention, two or more receiving antennas may be stacked above one another to provide an antenna system that can receive antenna signals from multiple bands with high gain. In a further aspect of the present invention, two or more passive antennas may be stacked upon one another to provide increased multipath suppression in multiple frequency bands. In yet a further aspect, an antenna system may comprise two or more receiving antennas stacked over one another to provide the benefits as described above, and two or more passive antennas stacked upon one another to provide the benefits as described above.
Accordingly, it is an object of the present invention to improve the down/up ratio of small microstrip antennas.
It is another object of the present invention to enable the construction of small antennas for receiving global positioning satellite signals which have the same or better multipath rejection performance as antennas with large ground planes or complex choke ring systems.
It is another object of the present invention to enable the bandwidth of microstrip antennas to be increased.
It is still another object of the present invention to enable the construction of an antenna which can receive both global positioning satellite signals and INMARSAT correction signals and/or other similar correction signals with good performance.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention, the accompanying drawings, and the appended claims.